KHEE-noh-PO-SAY-dahn, I think, it the correct Greek, but it actually depends on how the authors which to pronounce it. Note that KH is stressed as a hard K with a plosive (chi), rather than as in Lucy Lawless’ Zena + Poseidon.

Actually, the paper describing and naming Xeno nailed down how we intended the name to be pronounced, so there is really no excuse for the new NHM book to have made up its own version. See Taylor and Naish (2007:1549), where we say “Intended pronunciation: ZEE-no-puh-SYE-d’n.”